In conventional or “wet” lithographic printing, ink receptive regions, known as image areas, are generated on a hydrophilic surface. When the surface is moistened with water and ink is applied, the hydrophilic regions retain the water and repel the ink, and the ink receptive regions accept the ink and repel the water. The ink is transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Imageable elements useful as lithographic printing plate precursors typically comprise an imageable layer applied over the hydrophilic surface of a substrate. The imageable layer includes one or more radiation-sensitive components, which may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Following imaging, either the imaged regions or the unimaged regions of the imageable layer are removed, revealing the underlying hydrophilic surface of the substrate. If the imaged regions are removed, the precursor is positive working. Conversely, if the unimaged regions are removed, the precursor is negative working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and repel ink.
Imaging of the imageable element with ultraviolet and/or visible radiation typically has been carried out through a mask, which has clear and opaque areas. Imaging takes place in the regions under the clear areas of the mask but does not occur in the regions under the opaque areas. If corrections are needed, a new mask must be made. In addition, dimensions of the mask may change slightly due to changes in temperature and humidity. Thus, the same mask, when used at different times or in different environments, may give different results and could cause registration problems.
Direct digital imaging, which obviates the need for imaging through a mask, is becoming increasingly important in the printing industry. Imageable elements for the preparation of lithographic printing plates have been developed for use with infrared lasers. Although direct digital imaging has eliminated the mask, the equipment required for imaging, known as a platesetter, is expensive and can be complex, requiring, for example, computer controlled high intensity lasers.
Imaged imageable elements typically require processing in a developer to convert them to lithographic printing plates. Processing introduces additional costs in, for example, the cost of the developer, the cost of the processing equipment, and the cost of operating the process. However, on-press developable lithographic printing plate precursors can be directly mounted on a press after imaging and developed with ink and/or fountain solution during the initial press operation. These precursors do not require a separate development step before mounting on press. On press imaging, in which the precursor is both imaged and developed on press, eliminates mounting the precursor in a separate imaging device.
Thus, a need exists for a method for imaging a printing plate precursor that retains the advantages of using data in digital form and thus does not use a mask for imaging, yet does not require expensive and complex equipment. In addition, the precursors used in this method should be capable of being developed on press, so that neither a developer nor a separate development step is required. Preferably, the precursors should also be imageable on press.